Detection of heavy ions using Nb-based superconducting tunnel junction

The superconducting tunnel junction (STJ) is one of the most promising devices to use as a fast detector for heavy ions. The energy deposited due to the passage of a heavy ion through an STJ forms a region called a hot spot, where the superconductivity of the region is broken. As a result, a reduction of the critical current (I_c) in the STJ occurs. If the bias current exceeds this reduced I_c, the output voltage from the STJ switches from 0 V to its gap voltage, which is recognizable as a signal due to the heavy ion passage. Nb-based STJs were fabricated for this heavy ion detector and an experiment was performed at the RIKEN Ring Cyclotron Facility to investigate the response to heavy ions. Instantaneous switching to the voltage state of the STJ in response to the decrease in I_c induced by a heavy ion beam was successfully observed by introducing ⁴⁰Ar particles with a kinetic energy of 95 MeV/nucleon into the STJ